Design and regulation of high-performance photovoltaic systems based on novel two-dimensional KAgSe/KAgX (X = S, Te) van der Waals heterojunctions

Abstract

The realization of high-performance two-dimensional (2D) solar photovoltaic systems is both fundamentally intriguing and practically appealing for meeting increasing energy requirements. Considering the limited application of single 2D crystals in photovoltaics, here we propose a family of 2D KAgSe/KAgX (X = S, Te) van der Waals heterostructures (vdWHs), which are constructed by combining two different KAgX layers through interlayer vdW interactions. After a systematic study and further research on regulating the properties of these vdWHs based on first principles, numerous fascinating characteristics and physical mechanisms are observed. Firstly, the favorable potential applications of these vdWHs in photovoltaics are confirmed in consideration of their superior optoelectronic properties, such as robust stabilities, moderate direct band gaps, type-II band alignments, and superior carrier mobilities, visible optical absorptions, power conversion efficiencies (PCEs) and photocurrents in photovoltaic devices based on them. More importantly, when placed under varying vertical electric field (E_z), a phase transition of the band alignment from type-II to type-I can be induced in these vdWHs by opposite band shifts between layers, which may enrich their applications in lasers and light-emitting diodes. Meanwhile, the PCE can be increased up to 23%, and an obviously red-shifted peak in the photocurrent in the visible light range is also obtained at different E_z values. These fascinating tunable properties of KAgSe/KAgX vdWHs under varying E_z not only promote the improvement of their photoelectric performances, but the underlying mechanisms can also be applied to subsequent experimental synthesis and practical applications of other 2D photovoltaic systems. This is especially true for the red-shifted peak in the photocurrent, which is rarely found but highly desirable in practical visible photoelectric conversion.